Execution of plesk repair mail utility fails: mailsrv_conf_init... not found

Symptoms

Unable to access mailbox and send mail.

Execution of plesk repair mail fails:

# plesk repair mail
Reconfiguring all domains and mailboxes ......................... [2017-09-03 11:17:31] ERR [util_exec] proc_close() failed ['/opt/psa/admin/bin/mchk'] with exit code [1]
[FAILED]
- mchk failed: ==> Checking for: mailsrv_conf_init... not found,
skipped

Mail authorization database is empty:

# /usr/local/psa/admin/bin/mail_auth_dump
Authentication database contents:
+--------------------------------------+-----+--------------------------------------+
| address |flags| password |
+--------------------------------------+-----+--------------------------------------+
+--------------------------------------+-----+--------------------------------------+

In /var/log/maillog there are records like:

Sep 3 06:59:30 mail plesk_saslauthd[25101]: failed mail authenticatication attempt for user 'example' (password len=9) 
Sep 3 06:59:30 mail postfix/smtpd[25099]: warning: unknown[xxx.xxx.xxx.xxx]: SASL LOGIN authentication failed: authentication failure

Symbolic link /usr/bin/which points not to /bin/which

# ls -al /usr/bin/which
lrwxrwxrwx 1 root root 63 Aug 24 04:04 /usr/bin/which -> ../local/share/.config/yarn/global/node_modules/which/bin/which

Cause

Incorrect location of /usr/bin/which

Resolution

Change location of /usr/bin/which

# unlink /usr/bin/which
# ln -s /bin/which /usr/bin/which
# ls -al /usr/bin/which
lrwxrwxrwx 1 root root 10 Sep  4 04:15 /usr/bin/which -> /bin/which

Run plesk repair mail again.